I guess I never thought much of wiring L and R in an outdoor situation. I think you simply cause more cancellations that generate stereo imaging. I'd mono the signal myself and use half the speakers.

Remember, if they are using the main outputs for surround sound, the 'other' rooms will probably only get the main L and R effects etc. Plus, does the receiver 'borrow' the center or rear channel amplifier to generate the 2nd zone amplified sound?